The invention relates to probe heads, as for use in a coordinate-measuring machine (a) having a part which carries the probe head and is mounted for guided deflection in all directions in space and (b) having an associated measurement system for detection of the displacement involved in the deflection.
Such probe heads are of the "measuring" type. They make it possible to detect displacement of the work-contact ball of the probe pin relative to the housing of the probe head, in the course of a deflecting displacement, i.e., while in the work-contacting state, so that the computer of the measurement system can add the amount of the deflection, to the position-measurement value supplied by the scales of the measuring machine; the computer can thus determine the instantaneous position of the probe ball within the measurement volume of the instrument. These measuring-type probe heads are therefore particularly suitable for so-called scanning techniques, in which the workpiece to be measured is scanned by a probe pin which is in continuous sliding contact with the surface of a workpiece.
In contradistinction to the probe heads of the "switching" type which supply a trigger signal only at the time of the initial contact with the workpiece, "measuring"-type contact heads have at least three linear measuring systems, which may employ moving coils or photoelectrically scanned scales which are associated with the respective guides which movably mount the probe pin.
The guides of known probe heads of the measuring type are, as a rule, directed, like those of the coordinate-measuring machine itself, along the axes of a Cartesian coordinate system. Conventionally, these probe guides are built one on top of the other, i.e. the x-guide carries the y-guide, and the z-guide is carried by the y-guide.
Such a probe head is described in U.S. Pat. No. 3,869,799, wherein the respective guide systems are developed as spring parallelograms which are attached to each other. The probe head described in U.S. Pat. No. 4,611,403 also has, in principle, a very similar construction but in its case, the guides are made free of friction, using air bearings between parts which slide on each other.
As a result of the construction described, the movable masses are of different value in the individual coordinate directions since at least one of the guides must, after all, assist in carrying the guide parts for the two other coordinate directions. Accordingly, the dynamic behavior in the individual axes is very different, so that different forces occur between probe pin and workpiece, particularly in a rapid scanning operation. The precision of the measurement is impaired by such variations in measurement force.
U.S. Pat. No. 4,530,159 describes a probe head having a probe pin which is movably mounted with respect to two coordinate directions, via a flat-plane guide having two degrees of freedom. However, in this case, the third guide is carried by the flat guide, in the direction perpendicular to said plane; thus, once again, different movable masses are involved in the respective individual guidance directions. Furthermore, this probe head requires an additional means to secure against torsion, in order to foreclose an undefined rotation of eccentric probe pins in the plane of the guidance system having two degrees of freedom. Since this torsion restraint enters into a determination of the accuracy of measurement, it must be developed in very precise fashion, thus entailing considerable additional expense.